Ultrasonic cement analyzers (“UCA”) are well known in the art. A typical UCA provides a number of advantages over alternative methods for measuring or estimating the characteristics of a particular cement sample. A particularly compelling advantage is the ability of the UCA to perform nondestructive measurements at elevated temperature and pressure such as may be found in oil field applications.
The UCA was developed to measure the compressive strength of a cement slurry as the cement sets while subjected to oil field temperatures and pressures. A typical UCA consists of a high temperature, high pressure vessel, a heating jacket capable of heating rates up to 5.6° C. (10° F.) per minute, up to 204° C. (400° F.) and pressures to 138.0 MPa (20,000 psi).
A typical UCA utilizes a pair of ultrasonic transducers to measure transit time of an acoustic signal transmitted through the slurry as it sets. Set time and compressive strength are calculated from the measured transit time via empirically developed equations. U.S. Pat. Nos. 4,259,868 and 4,567,765 disclose UCAs in detail and are incorporated herein by reference.
The operating temperature of a typical UCA, however, is limited by the use of a ceramic with a Curie temperature of typically 300° C., a coupling coefficient of 0.71 and an impedance of 35×106 MKS Rayls. The limit on operating temperature of the standard UCA has typically been 200° C. With the need to evaluate cement at temperatures up to 316° C. (600° F.), a different type of piezoelectric ceramic is desirable.
Piezoelectric ceramics with a high Curie temperature are available. However, a problem associated with using piezoelectric ceramics in a UCA is that the typical coupling coefficient of the ceramic decreases as the Curie temperature increases. Because of the already low signal level and the lower sensitivity of high temperature piezoelectric ceramic, an improvement in the mechanical design of the UCA is desirable to increase the signal amplitude.
The piezoelectric ceramic in a typical UCA is mated to an interior of a plug at the bottom of a flat bottom hole approximately 2.5 inches deep. To achieve good signal amplitude in the measurement, the bottom of the hole should be extremely flat to couple acoustic energy from the transducer into the cement. Manufacturing and maintaining this flat surface presents difficulties. High temperature grease is typically used to compensate for inadequacies of the acoustic coupling. Over time this grease tends to degrade due to heat, which results in loss of signal. The thickness of the metal separating the piezoelectric ceramic from the cement acts as a narrow band filter that limits the measurement to a narrow range of frequencies.
It is therefore desirable to produce a transducer for use in a UCA that operates at higher temperature and produces higher signal amplitude to obtain accurate transit time measurements.